1. Field of the Invention
The present invention is related to a semiconductor optical device based on a semipolar substrate.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers within parentheses, e.g., (Ref. X). A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
State of the art Group-III nitride lasers are either grown on readily available c-plane sapphire substrates or on expensive SiC or bulk GaN substrates. All of them are grown along the polar [0001] c-orientation of (Ga,In,Al)N (also referred to as “Group-III nitride”, “III-nitride”, or just “nitride”). The laser diodes grown on sapphire usually employ dry-etched facets which lead to higher losses and consequently to reduced efficiency. The devices grown on SiC or bulk GaN substrates can have cleaved mirror facets.
However, laser diodes grown on various semipolar orientations have much lower polarization-induced electric fields as compared to those grown on the polar [0001] c-orientation (Ref. 1). Theoretical studies (Ref 2) indicate that strained InGaN/GaN MQWs grown on semipolar orientations are expected have significantly lower effective hole masses than strained c-plane InGaN quantum wells. This should lead to a reduction in the threshold of semipolar (Ga,In,Al)N laser diodes as compared to those fabricated on c-plane GaN. Thus, the cleaved facet, (Ga,Al,In)N edge-emitting laser diodes grown on semipolar {11-2n} III-nitride substrates of the present invention offer the promise of improved efficiency.
Recently, (Ga,In,Al)N edge-emitting laser diodes with {10-10} m-plane cleaved facets, fabricated on [0001] c-oriented bulk GaN substrates, have been reported (Ref. 3). However, no (Ga,In,Al)N laser diodes grown on semipolar {11-2n} (where n can assume any value, e.g., 2, −2) or semipolar {10-11} (where l can assume values such as 1, −1, 3, −3, etc.) bulk GaN substrates have been demonstrated.
Cleaved facet (Ga,In,Al)N edge-emitting laser diodes grown hetero-epitaxially on substrates, for example, a-plane sapphire, 6H—SiC, (111) spinel, etc. (Ref. 4-6), have also been previously reported. However, such laser diodes grown hetero-epitaxially on foreign substrates (e.g., a-plane sapphire, 6H—SiC, (111) spinel, etc.) exhibit inferior performance compared to those which are grown homo-epitaxially on bulk GaN substrates, due to the high defect densities arising from the large lattice mismatch between the epitaxial layers and the foreign substrate.
(Ga,Al,In)N based electronic and optoelectronic devices are mostly grown on GaN, which has a wurtzite crystal structure. For the wurtzite crystal structure, the (1-100) m-plane is perpendicular to the {11-2n} family of crystal planes, and is a naturally cleaving plane. This enables the present invention to grow edge-emitting laser diodes on semipolar {11-2n} III-nitride substrates, such as bulk GaN substrates, which employ atomically-smooth cleaved (1-100) m-plane mirror facets with high reflectivity.
In the prior art, (Ga,Al,In)N based edge-emitting laser diodes usually employ dry-etched facets, which are inherently rough. Moreover, it is difficult to achieve perfectly vertical facets using dry-etching techniques. This leads to scattering loss, and consequently, reduced reflectivity at the facets, resulting in inferior device performance.
The present invention combines the advantages of growing (In,Ga,Al)N devices on semipolar orientations (low polarization-induced electric fields in InGaN/GaN quantum wells (QWs), theoretically predicted lower effective hole masses compressively strained InxGa1-xN QWs, etc.), with the low loss cavity achieved by cleaved mirror facets, to enable the fabrication of superior performance devices.